| Summary: | 34th European Cetacean Society Conference, O Grove, 16-20 April 2023 Data collection is a challenge, especially when dealing with highly mobile species, as is the case of cetaceans, and surveying dynamic open ocean areas, which comes with several logistic constrains. Therefore, it is of extreme relevance to be able to use all available data to study the distribution and habitat of cetacean. The CETUS Project is a cetacean monitoring programme that uses platforms of opportunity to survey long line-transect routes in the eastern North Atlantic. The CETUS dataset contains data on cetacean occurrence collected both during dedicated monitoring effort and opportunistically (with interrupted monitoring effort). Here, we aim to compare opportunistic and dedicated data for the assessment of cetacean diversity, distribution and habitat. Due to the large amount of data collected over an 11-year period (2012-2022), we were able to compile sufficient opportunistic records to characterize the cetacean community in the area, but only for the most frequently sighted species. For frequent, easily detected and identified species (e.g., common dolphin, Delphinus delphis), the results for spatial distribution patterns and habitat range are similar when using dedicated and opportunistic data. On the other hand, for species that are less frequent, elusive and difficult to identify (e.g., Cuvier´s beaked whale, Ziphius cavirostris), there were important differences. For some of these species, there were several regions where the only available data on occurrence were collected opportunistically, and inclusion of such data would thus increase assessed distribution and habitat range. The results obtained evidence that there is high value of opportunistically collected data, and the rarer the species, the more relevant such data is. Ultimately, if possible, opportunistic occurrence data should be used in the analytical process and not lost No
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